A highly effective measure of reducing the unhealthy portions contained in the exhaust gas of an internal combustion engine, and more particularly for reducing the nitrogen oxide gas portions, is effected by recycling certain amounts of the exhaust gas. This addition of gases, which does not participate in the combustion process, produces a reduction of the combustion temperature, so that less nitrogen oxide occurs. However, this inert gas causes a reduction of cylinder filling with the air mixture, hereafter termed mixture at full loads, thereby adversely affecting the rotation of the engine at low rates of speed which is particularly evident during idling of the engine. Therefore, in order to maintain the maximum power output at full load, and to provide smooth rotation of the engine during idling, no recycling or return of exhaust gas should take place at there lower speeds of rotation. Moreover, the emission of exhaust gas is only limited during engine idling because the fuel rate and cylinder filling are both low; moreover, the combustion temperatures are lower, and accordingly also the emission of nitrogen compounds. In intermediate load ranges, however, the emission of nitrogen compounds is particularly high and therefore particularly dangerous because the engine usually operates in city traffic in (for which the emission control regulations are usually the most stringent) during intermediate load ranges. It is desirable, moreover, for the recycling of the exhaust gas to be interrupted temporarily during short-time full loads as they occur, for example, when passing another automobile. On the other hand, the emission control regulations are less stringent for highway driving which may require maximum power output under full loads without an accumulation of noxious discharge gases.
For satisfying the above requirements, a device is known from German published patent application (DOS) No. 23 38 906 which comprises the use of a vacuum amplifier for the purpose of providing a controlled vacuum for the EGR-valve. This device, in addition to requiring a vacuum supply reservoir, has the drawback that the amounts of exhaust gas returned within the lower load ranges at high suction or underpressures in the suction conduit are excessively high.